I'm telling you people you can forget all that and please don't kill me for it.
The key there is WATER, any water in the product instantly sets up for surface rust that appears in less than five minutes there. The WATER does it. I have used pool acid (muriatic or hydrochloric) but part needs to stay in long enough to remove the bad rust, that can often be long enough to damage close lying areas that were not rusted. The acid does not care, it eats both metal and rust at same time even though it goes after the rust a bit more.
Anything you can buy that mixes with water or 'biodegrades' is garbage unless an acid and the acid will depend on the strength used. You rinse the acid with water and an instant protection problem there, I used to use a Felpro product that sprayed on to form a thick waxy oil coat that would protect dead clean bare steel for up to 3 years, of course you had to remove it later and prep well after doing so.
Use vinegar (acetic acid) straight, it is already so low in strength it will not hurt at all, cutting it makes it useless, it already is before that. BTDT. You can drink it, do that with pool acid and you'll be dead in 3-4 minutes. Both acids can kill, the trick is in the concentration.
Pool acid at the strength supplied in bottles (30%?) is extremely dangerous stuff, I ruined an engine block once when someone suggested I fill water passages to remove rust. 3 hrs. later the acid ran out of side of block where it had cut a hole through water jacket. A 3/8"-24 bolt thrown in it for 30 minutes will no longer have threads on it. Now I use that stuff measured in 1-5 minutes, no more. DO NOT-I implore you-get it on your hands!!!!!!! By the time the pain starts (couple minutes) the damage is done. When I use it I have the water hose running within inches to dilute splatter that hits me.
I just dunked two cad plated pieces of steel in acid to remove the plating so I could braze it properly, they were both red rusty in 5 minutes airdry time and after blowing them off with high pressure air, which removes the water to slow that down. I have to re-prep them just before brazing but the cad is gone and why I did it.
Anything you can handle to put on side of car will not work very long before evaporation and the problem. The parts need to immerse in liquid, stay there, and just try that on a car. And in copious enough amounts to do some good, spreading a gel on there is not enough, it neutralizes and still part rusted. You'd have to use $50 worth to get anywhere. I bought a bunch once to wet dunk a bike head and remove paint from it, a mistake, the water soluable component in it then rusted all my fine parts like valve spring shims and anything else that fell to bottom of bucket. $30 worth of the 'best' paint removal stuff that could be bought. I was very upset, some of the parts were irreplaceable. It only sat overnight, maybe 12 hours and pulled water out of the air. Alcohols are what are commonly added to solvents to make them mix readily with water; I refer y'all back to my recent postings on ethanol. And how they've butchered paint removal chemicals to be dead worthless now.
I sold lots of this stuff at the store and have used a lot of it too, there is nothing over-the-counter available that does a GOOD job there. The liability issues have destroyed any usefulness of product, not that they were ever that great to begin with. I never found one person who was satisfied with the performance of those products, not one. They work on very thin rust, past that you are p-ssing in the wind. The lie is that you think you are getting somewhere but wait till you get to the end result. The 'turning black' literally means nothing, they ALL do that but the product will be on in such small amounts (not immersed, remember?) that the black simply means you have saturated the acid to neutralize it with still plenty of rust left. You will get sick of doing it twenty times and still getting black there, it never stops since at some point if true acid the black is coming from the base steel being dissolved and big mistake there.
I have not done a car in a long time but doing panels we had access to sandblasting or glass beading, which cleans it in like instant, the rust is not as hard as the steel and immediately comes up. Must use proper grit of product to produce the desired result. Dangerous (get some in eyes and it's a trip to emergency room, BTDT 3 times now for it) and messy though, but what the car restoring guys do. Kiss all that hand work goodbye....... ...the glass or sand gets in the microcracks that no sandpaper or grinder or stripping tool on the planet can. Look dead close at a blasted part in person and it becomes instantly evident. Even if you get all the rust off by handwork, you will always have the small pinhole rust spots, it continues to rust from those again. Thank God I do more bike restore than car, the parts are smaller and easy to make tanks to hold entire part to de-rust it. And I've spent a thousand dollars glass beading parts to do nothing else to them but wipe with alcohol and shoot paint on them.
Not trying to rain on anyone's parade but a dose of reality can save a bundle........ ......
Sorry dianne, I really don't hate you........... .............. .......
